There are many ways to classify ahadith. One method is by authenticity: is it saheeh (authentic beyond question), hassan (good), da'eef (weak), or mawdoo' (fabricated?)

In my answer on another question, I mentioned a hadith that's recorded in At-Tirmidhi, which is Hassan (according to my source). Although that example did not have a hadith which scholars derived a ruling from, it raised this question:

What kinds of ahadith do scholars derive fiqh from, in terms of authenticity?

I'm not interested in that discussion about tawaatur, so please don't mention that in your answer.

2 Answers
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There is no consensus as to exactly which hadiths are to be used to derive fiqh; that's one of the reasons (but hardly the only one) the madhahib are so often disparate in their rulings.

While the four major Sunni madhahib accept Sunnah as one of the major sources of Islamic law, second only to the Qur'an, they differed on exactly which ahadith properly reflected this Sunnah:

Hanafi: Not only did a hadith need to be sahih, it also needed to be mash'hur.

Maliki: Even sahih hadith could be rejected if they conflicted with customary practice of Medina.

Shafi'i: Only sahih hadith were to be used; no other conditions.

Hanbali: Only marfu' hadiths were to be used (even if they weren't sahih); da'if hadiths in general were also used in preference to qiyas.

This doesn't even get into the minor or non-Sunni schools, not to mention those scholars who follow no single madh'hab. And, of course, any two scholars can disagree on whether any given hadith is even sahih in the first place.

Scholars use only authentic hadith for fiqh. but problem is that each scholar has different method for determining if a hadith is authentic or not. a specific hadith is authentic in view of a scholar but is not authentic in view of another scholar and this leads to different fatwa about the same Islamic question. for example a scholar may consider any hadith in Bukhari hadith collection as authentic but another scholar may disagree this.

Authentic hadith is authentic only for scholar(s) who consider it authentic and not for all scholars.